1. Field of the Invention
The present invention relates generally to an electrophotographic image forming apparatus and method, and more particularly to an image forming apparatus and method of a type controlling an exposure output upon correction of gradations of image data.
2. Description of the Related Art
In a digital color copier, image signals from an image reader such as a CCD or the like are initially converted to digital image data of, for example, eight bits (256 gradations) by an analog-to-digital converter (A/D converter). Thereafter, in order to enhance the fidelity of half-tone images, an exposure output is controlled upon correction of gradations of the digital image data based upon a gamma-correction table formulated in accordance with non-proportional gradation characteristics called the "gamma-characteristics" between the image data to be reproduced and reproduced image density levels. The gamma-correction table is made in advance based upon a relationship between the image density and the image data and is stored in a ROM or the like.
Meanwhile, in a color copier, because color reproduction is performed based upon the subtractive color mixing of toners of cyan, magenta and yellow, any desired color reproduction needs the stability of the image density (the amount of adhesion of toner) of three different colors with respect to a photosensitive member.
To this end, an automatic toner density control (ATDC) and an automatic image density control (AIDC) are both performed in the apparatus. More specifically, in the ATDC, the toner density in a developer is detected by a toner density detector means and the amount of toner to be replenished is controlled in accordance with a detection signal thereof, thereby making the toner mixture ratio in the developer substantially constant. On the other hand, the AIDC is performed to adjust the apparatus to changes in the amount of electrostatic charge of the developer or characteristic changes of the photosensitive member caused by environmental changes. In this control, the density of a reference pattern formed on the photosensitive member is initially detected and developing conditions are changed in accordance with a detected density signal, thereby making the amount of adhesion of toner (the toner density) substantially constant. A method of changing a developing bias is widely used with highest reliability as a method of changing the developing conditions.
However, when the developing conditions are changed according to the environmental changes or the like, the gradation characteristics also change. Because of this, a single gamma-correction table cannot provide the optimum gradation correction, and therefore, the most appropriate gamma-correction table is selected from among a plurality of gamma-correction tables according to the developing conditions.
In this kind of image density control, half-tone images are faithfully reproduced with an appropriate density when the toner mixture ratio is always constant. Even though the toner mixture ratio is made constant by the ATDC, the actual toner mixture ratio always changes according to the area ratio (B/W ratio) between an image portion and a white ground of an original document or copying modes such as, for example, a continuous copying mode or an intermittent copying mode. In other words, even though the toner mixture ratio detected by an ATDC sensor is fed back to a toner replenishing means, it is impossible to immediately replenish, agitate and transport the toner. As a result, the toner mixture ratio changes.
Meanwhile, a central value in controlling the toner mixture ratio is occasionally changed. In the image density control by the AIDC, the grid voltage and the developing bias are set low when the humidity is comparatively high. As a result, the 256 gradations of the density must be controlled by a low developing voltage, and changes in surface potential or in the amount of light are followed by wide variations in the density. In contrast, the grid voltage and the developing bias are set high when the humidity is comparatively low. Since these values have respective upper limits due to the pressure resistance of the photosensitive member, saturation of the developing efficiency, or the like, the central value in controlling the toner mixture ratio is occasionally switched according to localities or seasons.
When the toner mixture ratio changes, the image density is controlled to be constant by the AIDC. However, since the gamma characteristics change as shown in FIG. 5, desired images cannot always be obtained merely by selecting a gamma-correction table from among a plurality of tables in compliance with the AIDC.